Mr. H. G. Seeley on the Potton Sands. 27 
Not having seen Mr. Walker’s specimens, I am unable to 
speak with confidence on the other species named ; but no such 
shells as Exogyra conica, Modiola equalis, and Myacites plicata 
have come under my notice, though I have long had other spe- 
cies of those genera in the Woodwardian Museum. 
X. The author’s list of fish and reptiles needs but brief com- 
ment, the names being in part identical with those which have 
for years been attached to similar fossils in the Woodwardian 
Museum ; but it can hardly be necessary to assure any one that 
the genera Pycnodus, Hybodus, Lepidotus, Gyrodus, &c. are 
just as little found only in the Kimmeridge Clay as are the spe- 
cies Asteracanthus ornatissimus and Lepidotus (Spherodus) gigas, 
and that there can be no reason for thinking them other than 
tenants of the sea of the time. Had the author availed himself 
more fully of the collections to which he appears to have had 
access, he might have chronicled a more wonderful series of 
fossils than those enumerated—a series as rich perhaps in 
genera and species of fossil reptiles as any known geological 
fauna, 
XI. The author quotes the existence, in the Woodwardian 
Museum, of shelly limestone containing Cyrene, and uses this as 
evideuce for inferrmg some of the fossils to have been derived 
from the Wealden. I can confidently say that no such speci- 
mens have ever been found; and the concretions which were 
supposed to be the said shelly-limestone, on being broken, are 
found full of Cardium, Cytherea, &c.- Moreover I have shown, 
in my paper on these beds, that the material of the deposit 
came from the east. 
XII. Finally, Mr. Walker has described and figured (pl. 13) 
two shells. The one referred. to Sphera Sedgwickii is not a 
Sphera, but a Cyprina, and only differs as a variety from C. 
angulata (Sow.), a type prolific in varieties. The form figured 
is not typical. The species referred to Pholas Dallasii may be 
new. As every one is aware, all the secondary Pholades belong 
to the genus Pholadidea. This species burrows in wood, and 
lines its burrow with shell, and rather approximates to Xylophaga 
and Teredina than to Pholas. It has no affinity to D’Orbigny’s 
P. Cornueliana. 
The age of the beds to which Mr. Walker’s paper relates is a 
difficult problem, and not one that can be solved by an appeal 
to fossils, or mineral character, or superposition. And it is in- 
timately bound up with questions of great interest, such as the 
age of the Farringdon beds and the nature of the marine equi- 
valents of the Purbeck and Wealden strata. For I have found 
to the north of Cambridge most of the Farringdon fossils in a 
